Adults with serious mental illness (SMI) have a 20 percent reduced life expectancy and disproportionately greater prevalence of medical comorbidity associated with high rates of obesity, sedentary lifestyle, and poor dietary and health behaviors. This is a application for a randomized controlled trial evaluating the effectiveness of a novel health promotion and fitness intervention (In SHAPE) specially tailored for, and pilot tested among persons with SMI. Study participants will include 200 adults with SMI stratified by diagnostic group (n=100 schizophrenia-spectrum disorders and n=100 mood disorders) and randomized to either: a) In SHAPE, consisting of a personal fitness assessment and development of diet and exercise goals, weekly meetings with a health mentor, monthly group sessions on diet and weight management, a YMCA membership, an incentive program, and group educational and motivational sessions; or b) a health club membership and educational materials (HCME). In SHAPE participants will receive a one-year intensive program (weekly contact with a health mentor), followed by a 6-month maintenance phase (bi-weekly meetings tapering to monthly meetings with a health mentor), with assessments conducted at baseline, 3, 6,12 and 18 months. This study will address the following 3 specific aims: Aim 1) To compare the effectiveness of In SHAPE and HCME with respect to physical fitness outcomes, including: (a) engagement in regular exercise and dietary change and (b) indicators of physical fitness, including performance on the 6-minute walk test, decreased waist circumference, and decreased BMI; Aim 2: To compare the effectiveness of In SHAPE and HCME with respect to mental health outcomes, including negative symptoms, depressive symptoms, and self-efficacy; and Aim 3) To explore the effectiveness of In SHAPE and HCME with respect to psychosocial functioning, health status, and acute service use, as well as the effects of selected demographic, clinical, and health behavior variables on primary outcomes. If found to be effective, the In SHAPE program could provide a model for improving fitness, dietary behaviors, and mental health outcomes for this vulnerable and at-risk population within public sector mental health settings. The potential downstream impact on morbidity and mortality has public health implications for large numbers of persons with SMI.